1. Field of the Invention
The present invention relates to a bone fixation assembly and to a transverse linking bridge for spinal fixation assemblies, the invention being related in particular to the field of the devices widely used for correcting the position of the spine or spinal column.
2. Description of the Prior Art
It is well known, under some circumstances--due to natural malformations or surgery--that it is necessary to correct the deformation or incorrect position of the spine to bring the spine back to its natural and correct position. The spinal column preferably has an extension along a vertical line as seen from the front of a human being and a "S" shape as seen from the side. Due to different circumstances the mentioned "S"-form as well as the vertical line are deformed because of a displacement of the vertebrae one with respect to the other. This deformations may produce displacements, fractures, tumors and infections which could be prevented correcting the alignment of the vertebrae through the use of devices that compress, distend or rotate all or parts of the vertebrae that form the spinal column in relation to one or all spatial reference axes X, Y and X.
Several mechanical devices or arrangements or assemblies have been developed to this end. Generally, all of these consist of sets of screws and rods, as well as fixation plates that form very complex mechanical systems. The older ones typically used a rod or rigid bar that fixed the spine by means of screws inserted in the pedicles to fasten the bar. Then the rod is bent to the desired arrangement according to the corresponding axis to bring the column into its normal position. In all cases, the screws fixed to the vertebrae must allow the fixation of one or more rods that have to pass through and be threaded in some type of connector to each one of the screws. It is easy to see that it is very difficult to make a rigid bar pass through a plurality of screws that probably will not be aligned in the desired position and that will also not show its connector receptors oriented in alignment to allow the passing through of a bar or rod. This is very difficult when the corrections at the spinal column must be made in the three dimensions of space, i.e. with respect to the three axes X, Y and Z. Thus, the screws that are fixed over the vertebrae usually have to be removed and placed again to find a new orientation, and this may cause a considerable damage to the bone tissue of the vertebrae in which the screw is being threaded. Sometimes there is not enough space to make this reorientation and this means a risk for the patients health because there exists the possibility of fracturing the most fragile parts of the vertebrae with the resulting consequences.
Among the several fixation arrangements and assemblies for the spinal column it can be mentioned the one disclosed in U.S. Pat. No. 5,498,262 which defines an assembly consisting of a pair of bars that pass through coupling devices rigidly fixed by means of a screw to the bar while at the other end they are fixed to a clamp through which rigidly passes a screw threadably inserted into the corresponding vertebra. In the case of the fixation between the connector and the rod as well as in the case of the fixation between the connector and the bone fixation screw, i.e. the one that is inserted in the vertebra, the threaded holes allow the insertion of the screw only in one position, which makes the assembly of the arrangement in the spinal column too rigid with a reduced margin of movement, and accordingly, with a reduced margin of error for the surgeon while positioning the screws.
Other devices with some improvements relating to the assembly of the fixing screw, e.g. in inclined form, are provided, like the one disclosed in U.S. Pat. No. 5,306,275, in which the fixation of the screw in relation to the extended bar between screws is rigidly obtained by means of a perforated stem which receives the bar and which fixes the same against the screw by means of a nut. However, as already mentioned, the connection is rigid not allowing an omni-directional movement.
U.S. Pat. No. 5,129,900 provides an vertebrae fixation arrangement for the spine that includes a coupling part presenting at one end a receptor bore of one of the spine correcting bars, which is secured inside the connector part by means of a fastener or screw while at the other end it presents a substantially elongated opening for passing through a bone fixation screw. This elongated opening allows the bone fixation screw to accommodate along the axis of this opening, whereby the distance between the fixation screw and the rod may be variable according to the length of this opening. However, this elongated opening does not allow the accommodation of a screw that has been positioned in angular o deviated form with respect to the straight line passing through any longitudinal point of said opening. That means, that if a bone fixation screw would have been situated in deviated form with respect to its pre-established path, the longitudinal geometrical axis of the screw would not pass through the opening foreseen in the connection part due to an angular difference.
To accommodate angular positions of the bone fixation screws, some connecting assemblies provide beveled washers in different angles so that the inclination of the screw does not hinder the tight fastening of the coupling member, allowing the nut in the screw to integrally rest on and secure the fastening screw and the coupling member to the rod of de spinal fixation. Again, this is a device that allows only a good seat between the nut of the screw and the fastening element provided that the screw may accommodate its inclination inside the opening of the fastening element. Even so, the provision of nuts and washers with inclined surfaces only makes more complicated the assembly of the arrangement and provides too many parts that are not very desirable for the safety of the patient.
In the fixation assemblies in which two paralell bars fixed to both sides of the spinal column vertebrae are used, the bars are connected one with respect to the other by means of interconnecting transverse bridges. This interconnecting bridges present different arrangements which allow them to vary their length in order to accommodate to the spacing of the longitudinal bars of the assembly, as well as to any angular difference or pivot movement of the bars with respect to a transverse axis of the same. That means, there are bridges consisting of two telescopic parts that may pivotally move one relative to the other. The bridges may be elongated by means of these telescopic parts that are adjustable one with respect to the other or by means of extensible parts with threaded rods of nut and counternut with a central hub for the contraction and expansion of the bridge parts. These type of bridges are disclosed, inter alia, in the U.S. Pat. Nos. 5,522,816, 5,084,049 and 5,498,262. However, neither of these interconnecting transverse bridges between corrective bars of the spinal column allow to adjust the bars one with respect to the other modifying not only its spacing, but also its rotation with respect to a transverse axis to the same and its angle of difference of parallelism, i.e., the variation of the angle in the plane that contains the same.
In view of the above mentioned, it would be desirable to have a bone fixation element for spinal column fixation arrangements that would offer more possibilities of assembling the arrangement by means of an multi-directional adjustment in its connection with the arrangement, as well as to have a transverse interconnecting bridge for the bars of the spine fixation arrangements that would allow also a not so rigid assembly with respect to the interconnection of the different parts of the fixation arrangement.
3. Summary of the Invention
It is therefore one object of the present invention to provide a bone fixation device for spine fixation assemblies that allows to connect more freely the bone fixation element to the bars of the fixation assembly, even in spite of the position deviations between the same, deviations that would difficult the alignment and interconnection of each connecting part of the arrangement, as is the case in the previous art.
According to an aspect of the invention, a bone fixation element is provided that allows to accommodate more easily each of the parts forming the bone fixation assembly at the moment of the interconnection of the same, whereby the fixation element comprises a hook or screw that presents an intermediate portion of spherical form that may be coupled with a seat portion of a coupling piece between the fixation element and a bar of the fixation assembly.
More precisely, it is an object of the invention to provide a bone fixation element for a spine fixation assembly of the type which comprises a screw or hook with a threaded portion of fixation to the bone, vertebrae or the like, whereby the element may be connected to a coupling piece by means of a fastening nut, while the coupling is fixed to a bar of a pair of bars of the spine fixation arrangement, said fixation element presenting an intermediate portion at least partially spherical and a top portion for the threading of said nut, and said coupling piece presents a portion of receptor seat at least partially spherical, which receives said at least partially spherical portion of screw, said seat portion presenting a passing through bore for said upper portion of the fixation element.
Another object of the present invention is to provide a transverse interconnecting bridge for bars of spine fixation assemblies that allows to connect the bars transversally one with respect to the other, even in spite of the position deviations in each plane or angle between the same, deviations that would make difficult the alignment and interconnection of each of the bars and the other connecting parts of the arrangement.
According to one aspect of the invention a transverse interconnecting bridge is provided between the bars of the spine fixation assembly, the bridge including one or more parts that have at its ends a coupling hook in which a pivoting spherical insert is located which insert accommodates the deviations of the respective bar at the time the fixation is carried out.
More specifically, the invention provides a transverse interconnecting bridge for bars of a spine fixation assembly, of the type that includes only one part or at least two parts that may be coupled to form a bridge of adjustable length, the bridge presenting ends in form of hooks to allow the coupling to respective bars of the fixation assembly, one of said ends in form of hooks presenting at least an insert of outer surface at least partially spherical pivotally mounted inside of said hook, said insert presenting a fitting inner surface for each of said bars.
The above and other objects, features and advantages of this invention will be better understood when taken in connection with the accompanying drawings and description.